Carbon monoxide is typically produced and recovered from synthesis gases from one of several well understood reforming methods including: steam-methane reforming, auto-thermal reforming, oxygen reforming, dry carbon dioxide reforming, partial oxidation and combinations of the above reformation reactions.
The synthesis gases produced from these reactions typically contain hydrogen, carbon monoxide, carbon dioxide, methane, water and possibly nitrogen and argon.
This synthesis gas is usually cooled in several heat exchangers to raise steam, preheat reformer feed, preheat boiler feedwater and heat makeup water. The cooled reformer synthesis gas then enters a typical carbon dioxide removal system wherein carbon dioxide is removed from the synthesis gas. The carbon dioxide removal system typically consists of chemical absorption of carbon dioxide into a liquid solvent, which is regenerated in a stripping column. This system uses a solvent selected from the group consisting of MEA, MDEA, Rectisol, Benfield or other well known systems in the prior art.
In the case of recovery of CO alone, the carbon dioxide stripped synthesis gas leaving the carbon dioxide removal system would enter a thermal swing adsorption (TSA) drier where water and residual carbon dioxide are removed down to part per billion (ppb) levels. Traditionally, this is achieved using zeolites, like 13 X-zeolite. Often a 2 bed system is used where one bed is on stream for a number of hours, while the other bed is being regenerated with hot gas, then cooled down to feed temperature. Three bed systems can also be employed. The water and carbon dioxide-free synthesis gas then enters a cryogenic distillation system where pure carbon monoxide is recovered. It is important to remove water and carbon dioxide to trace levels to avoid plugging the cryogenic distillation system.
The removal of water and trace amounts of CO.sub.2 from various gas mixtures is the subject of much prior art. A vast majority of the prior art deals with the pre-purification of air prior to cryogenic distillation. Both pressure swing adsorption (PSA) and thermal swing adsorption (TSA) processes are taught.
Thermal swing adsorption processes for carbon dioxide and water removal from air are disclosed in U.S. Pat. Nos. 2,968,160 and 4,030,896. In these systems atmospheric air is compressed to about 100 psia followed by water cooling. The air is then further cooled using refrigerated ethylene glycol to remove water by condensation. The gas is then passed to a molecular sieve bed where the remaining carbon dioxide and water are removed by adsorption. The sorbent beds are operated in thermal swing mode with equal periods for adsorption and regeneration (heat-up, depressurization, cool-down and repressurization).
U.S. Pat. No. 4,249,915 discloses a process where moisture and carbon dioxide are removed from atmospheric air by adsorption in separate beds. The moisture removal bed (filled with a solid adsorbent effective in the adsorption of water) is regenerated by pressure swing adsorption in a relatively short operating cycle, while the carbon dioxide laden bed (filled with an adsorbent effective in the retention of carbon dioxide) is regenerated thermally at considerably longer time intervals.
U.S. Pat. No. 3,841,058 describes a 2 bed process for the removal of water, methanol and carbon dioxide from natural gas. The first bed contains an adsorbent for removal of water and methanol which is thermally regenerated at elevated temperature (about 300.degree. C.). The second bed contains an adsorbent for carbon dioxide removal which is regenerated at temperatures not exceeding 100.degree. C.
U.S. Pat. No. 5,531,809 teaches the use of 3 A-zeolite to dry synthesis gas as a pretreatment layer in a vacuum swing adsorption (VSA) system to produce high purity CO. The 3A layer is placed on the feed end of the adsorption bed.
U.S. Pat. No. 4,732,596 discloses a process for the co-production of carbon monoxide and hydrogen. The prior art method for production of CO is given in FIG. 1 of that disclosure. It shows the presence of driers after the MEA carbon absorbent for removal of water and carbon dioxide. Typically these system are TSA's filled with appropriate zeolite like 4 A-zeolite or 13 X-zeolite, which are capable of both water and carbon dioxide retention.
The prior art has not recognized a problem that occurs during the repressurization of adsorbent used to dry and remove carbon dioxide from synthesis gas, wherein water and carbon dioxide can reform from the synthesis gas used to repressurize the adsorbent, and the prior art has not provided the solution to this previously unrecognized problem, which problem and the present invention's solution to the problem will be set forth in greater detail below.